Olefin separation with a tetraalkyl urea



United States PatentO 'OLEFIN SEPARATION WiTH A TETRAALKYL URE Earle C. Makin, Jr., El Dorado, Ark., assignor to Monsanto Chemical Company, St. Louis, Mo., a corporation of Delaware No Drawing. Filed Oct. 15, 1958, Ser. No. 767,266

12 Claims. (Cl. 183-115) This invention relates to the. separation of mixtures of hydrocarbons and more particularly to the separation of mixtures of aliphatic hydrocarbons containing less than six carbon atoms.

Many refinery operations result in the production of a mixture of aliphatic hydrocarbons containing less than six carbon atoms and composed of parafiins and olefins, either monoor diolefins. The olefinic component of such streams is most useful in the preparation of polymeric materials. Frequently, the olefinic component of such streams can be used without complete separation from the paraflinic components if its concentration can be raised to a suitable level.

It is an object of this invention to provide an improved method for concentrating the olefinic content of hydrocarbon streams composed of mixtures of parafiins and olefins containing less than six carbon atoms. Additional objects will become apparent from the description of the invention.

It has now been discovered that the olefinic content of a hydrocarbon stream comprising a mixture of paraffins and olefins containing less than six carbon atoms can be increased through the selective solvent action of tetraalkyl ureas. Specifically, this invention comprises contacting a feed mixture of aliphatic hydrocarbons containing parafiins and olefins each containing less than six carbon atoms with a tetraalkyl urea and recovering from said tetraalkyl urea a hydrocarbon mixture having a higher olefin content than said feed mixture.

The tetra-alkyl ureas have been found to have a remarkable selective solvency for mono-olefins and diolefins containing less than six carbon atoms as compared with parafiins containing less than six carbon atoms. This unusual phenomenon is illustrated in Table I which sets forth the Bunsen coefficients for butadiene and butenes in several tetraalkyl ureas as compared with dialkyl ureas. The Bunsen coefiicients represent the volume of hydrocarbon dissolved per volume of solvent at 25 C. and 760 mm. Hg absolute".

'Percent Butadiene 2921 Butanes 50.5 Butenes 182 C aliphatics 119 C aliphatics 0.3

This hydrocarbon mixture was then bubbled. through tetraetnyl urea and tetra-n-butyl urea at a gas-to-solvent volume ratio of 40:1 at a solvent temperature of 26 30 C. In each case the rafiinate and extract phases were then analyzed for hydrocarbon content and. theresults are set forth in Table II.

1 Raffinate and extract refer to volume percent distribution of the feed gas as unabsorbed and dissolved hydrocarbon, respectively.

The results set forth in this example clearly illustrate the outstanding utility of tetraalkyl ureas as a means for solvent concentration of the olefinic content of mixtures of hydrocarbons containing less than six carbon atoms. Mixtures of hydrocarbons containing six or .more carbon atoms cannot be resolved through the use of these solvents for the reason that paraflinic and olefinic hydrocarbons containing six or more carbon atoms are completely miscible with the solvents. I

While any tetraalkyl urea, orrnixtures. of tetraalkyl ureas, can be used in the process of this. invention those wherein. the alkyl groups each contain not more than eight carbon atoms are preferred: Tetraalkyl ureas wherein each alkyl group contain as many as 18 carbon atoms can be used if desired. Typical of the tetraalkyl ureas that can be used in the process of this invention are those set forth in Table III.

Table III Boiling Urea Point, Pour Point,

CJmm F.

Hg Abs 'Ietraethyl urea 205/760 81 1,1-Diis0amyl-3,3-diethy1 urea. 134-5/4. 8 88 Tetra-n-butyl urea 140-5/3. 1 88 1,1-Dicyc1ohexy1-3,3-diethyl urea 904/1. 4 -5 1,1-Diethyl-3,3-di-n-octy1 urea 174-9/3 80 1,l-Di-nbutyl-3,3di-2-ethylhexyl urea 1734/1 -47 1,1-Di-n-buty1-3,3-di-n-0etyl urea. 187-8/2 81 Tetra-n-hexyl urea 1804/2 69 1,1- Dicyclohexyl-3.3-di-n-0ctyl urea. 217-34/2 -35 1,1-Di-n-hexy1-3,3di-n0ctyl urea 190-8/2 20 1,1-Di-n-butyl-3,3-di-n-dodecyl urea 230-57/2 -30 Tetra-2ethylhexyl urea 212-15/2 10 1,1-Di-2-ethylhexyl-3,3-di-n-0ctyl urea 209-12/2 59 Tetra-n-octyl urea 222-28/2 +5 1,1-Di-n-d0decyl-3,3-di-n-hexyl urea 250-/2 40 1,1-Di-n-dodeeyl-3,S-di-Q-ethylhexyl urea 200-45/7 -28 1,1-Di-n-dodecyl-3,3-di-n-0ctyl urea.-. 265-75/2 +14 1,1-Di-n-butyl-3,3-di-n-ootadecyl urea.- 297-305/2 +40 1,1-Di-n-hexyl-3.3-di-n-0ctadeeyl urea 290-300/2 +25 1,1-D1-n-0ctyl-3,3di-n-octadecyl urea 305-15/2 +35 This unusual behavior of the tetraalkyl ureas permits their use in the concentration of olefins in mixtures of aliphatic hydrocarbons containing less than six carbon atoms. This utility is illustrated in the following example:

A feed gas was prepared having the following composition expressed as volume percent can contain paratfins containing from 1 to 5 carbon atoms, mono-olefins such as propylene, butenes, and pentenes and diolefins such as propadiene, butadienes and pentadienes. The concentration is etfected by any of the conventional techniques for vapor-liquid extraction Well known to those skilled in the art. Preferably, the hydrocarbon mixture in the vapor state is merely bubbled through the liquid tetraalkyl urea. The gas-tosolvent volume ratio can be varied substantially with ratios ranging from :1 to 60:1 being preferred. The temperature of the solvent extraction step can also be varied with temperatures ranging from 0 C. to 100 C. being applicable. The process is very conveniently carried out at approximately room temperature. The temperature, of course, will be governed to a large extent by the fluidity of the solvent at the temperature employed, the boiling point of the hydrocarbon mixture and the pressure at which the solvent extraction is carried out. Atmospheric pressure has been found to be very suitable although pressures either below or above atmospheric can be used if desired. Single or multiple stage solvent extraction can be used as desired.

The hydrocarbon mixture concentrated in the extract phase can be recovered by any method well known to those skilled in the art. A simple distillation is usually the preferred method. The hydrocarbon streams recovered from the extract phase and the raifinate phase can be further processed in any desired manner.

What is claimed is:

1. In a process for separating olefins from a feed mixture of aliphatic hydrocarbons containing parafiins and olefins each containing less than six carbon atoms, the step of extracting said mixture with a tetraalkyl urea.

2. In a process for separating olefins from a feed mixture of aliphatic hydrocarbons containing paralfins and olefins each containing less than six carbon atoms, the step of contacting said feed mixture with a tetraalkyl urea to form an extract phase and a raffinate phase and recovering from said extract phase a hydrocarbon mixture containing a higher concentration of olefins than contained in said feed mixture.

3. In a process for separating olefins from a feed mixture of aliphatic hydrocarbons containing paraffins and olefins each containing less than six carbon atoms, the step of contacting said feed mixture maintained in the vapor phase with a liquid tetraalkyl urea to form an extract phase and a rafiinate phase and recovering from said extract phase a hydrocarbon mixture containing a higher concentration of olefins than contained in said feed mixture.

4. The process as described in claim 3 wherein each alkyl group of the tetraalkyl urea contains from 1 to 18 carbon atoms.

5. A process as described in claim 3 wherein each alkyl group of the tetraalkyl urea contains from one to eight carbon atoms.

6. A process as described in claim 5 wherein the gasto-solvent volume ratio is maintained in the range of from 10:1 to :1.

7. The process as described in claim 6 wherein the tetraalkyl urea is tetraethyl urea.

8. The process as described in claim 6 wherein the tetraalkyl urea is tetra-n-butyl urea.

9. In a process for separating butadiene from a feed mixture of aliphatic hydrocarbons containing butadiene and paratfins containing less than six carbon atoms, the step of contacting said feed mixture in the vapor phase with a liquid tetraalkyl urea to form an extract phase and a rafiinate phase and recovering from said extract phase a hydrocarbon mixture containing a higher concentration of butadiene than contained in said feed mixture.

10. A process as described in claim 9 wherein the gasto-solvent volume ratio is maintained in the range of from 10:1 to 60:1.

11. A process as described in claim 10 wherein the tetraalkyl urea is tetraethyl urea.

12. A process as described in claim 10 wherein the tetraalkyl urea is tetra-n-butyl urea.

References Cited in the file of this patent UNITED STATES PATENTS 2.445,520 Francis et al July 20, 1948 2,456,723 Beach Dec. 21, 1948 2,824,859 'Fasce Feb. 25, 1958 

1. IN A PROCESS FOR SEPARATING OLEFINS FROM A FEED MIXTURE OF ALIPHATIC HYDROCARBONS CONTAINING PARAFFINS AND OLEFINS EACH CONTAINING LESS THAN SIX CARBON ATOMS, THE STEP OF EXTRACTING SAID MIXTURE WITH A TETRAALKYL UREA. 